Exactly where the ductwork leaves the compartment from which it serves (demonstrated in blue) it must maintain integrity and insulation fire ratings for a similar period of time because the compartment through which it passes. Promat has made self-supporting programs without a metal liner, for example PROMATECT®-AD, PROMATECT®-L500 and MASTERBOARD®, https://daltonpfujz.blogsidea.com/43370447/about-grease-tight-extract-ductwork